One night, a fair old trot into the film, they chance upon a man hanging under a bridge on the Thames. It's an amnesiac played by Heath Ledger, and the endless morbidity of the ensuing dialogue – "Is he dead?", "We must try to bring him back to life", etc – does take your breath away a bit. It'll pass, you think, it'll pass.

But as the film goes on, and on, and on, the whole lot of them knocking around unproductively in the shell of Battersea power station, trying to stop the devil (Tom Waits) from escorting Lily into hell on her 16th birthday, it soon becomes apparent that, actually, the logistical ripples of Ledger's untimely death may well provide the only flickers of interest the film can ignite.

As everyone presumably now knows, Ledger's role was picked up by a trio of other actors who take turns filling his shoes. Hence we have Jude Law, Johnny Depp and Colin Farrell all, very creepily, dolled up to look as much like Ledger as possible.

The first time this happens – with Johnny Depp – you assume, that's it, then: that Ledger's final scene has now been shown. Why else the strange moment in which Depp looks with great surprise at his face in the mirror? Or the odd speech in which Depp consoles a woman upset that artistic icons – James Dean and Rudolf Nureyev are namechecked – sometimes die young?

But then, bang – Ledger is back again for more irritating burlesque. Then, suddenly, he's morphed into Law. Then a bit more Ledger. Then a whole lot of Farrell. It's understandable, of course – films are rarely shot sequentially. But one's thoughts, inevitably (and especially, given the less than gripping narrative) do wander toward unsavoury speculation: did they rehearse this scene with Ledger before he died? Was this one shot in the week beforehand? Yes, he does look like he was having trouble sleeping.

It's sad, upsetting even, at this distance. But Imaginarium mightn't have been a vastly better end product had Ledger been in every intended scene. Few films are so disjointed that you can just insert other actors when your leading man passes away. It might even have been worse: an unremarkable turkey, rather than a piece of work given more coincidental power and import than it perhaps deserves.